1. Field of the Invention
The present invention relates to a system and method for canceling a periodic noise from an original input signal to be processed including such periodic noise components.
2. Description of the Prior Art
A conventional system for canceling a periodic noise from a primary signal including a periodic noise using a known synchronous adaptive noise canceling means is exemplified by a Japanese paper named "Papers of Society of Instrument and Control Engineers" volume 19, No. 3, pages 34 to 40, published on Mar. 20, 1983, and titled to "A synchronous Adaptive Noise canceling System for Periodic Interferences" by Hidefumi Kobatake.
FIG. 1 diagrammatically shows the synchronous adaptive noise canceling system disclosed in the above-identified Japanese document.
In FIG. 1, synchronous adaptive noise canceling means 1 produces sequentially a periodic noise component y(t) at each phase of the periodic noise in synchronization with a reference pulse signal X(t) having a correlation to the periodic noise included in an original signal d(t) to be processed in terms of its period. Then, the noise component y(t) is subtracted from the original signal d(t) to be processed to produce a target signal component e(t) included in the original signal d(t).
A specific operation of the above-described noise canceling system will be described below.
Assume d(t) shown in FIG. 2(A) as the original signal to be processed (it is noted that a real signal component included in the original signal d(t) to be processed has a "0" level (substantially zero voltage) for explanation conveniences).
The synchronous adaptive noise canceling means 1 produces the noise component yk(i) having a phase k at the present period in accordance with such an equation as shown below from a noise component yk(i-1) produced at a previous period and the target signal component ek(i-1) fedback and produced at the previous period. EQU yk(i)=yk(i-1)+.alpha..multidot.ek(i-1)
wherein .alpha.: correction coefficient and 0&lt;.alpha.&lt;1 and .multidot. denotes a multiplication.
The target signal ek(i) is consequently derived in accordance with the following equation. EQU ek(i)=dk(i)-yk(i)
That is to say, assume that yk(0)=0.
As appreciated from FIG. 2(B), the following equation is established.
ek(0)=dk(0) (a state in which no periodic noise is eliminated)
Thereafter, the synchronous adaptive noise canceling means 1 sequentially corrects the noise component and accordingly corrects the target signal as expressed in the following equations. EQU yk(1)=yk(0)+.alpha..multidot.ek(0) EQU ek(1)=dk(1)-yk(1) EQU yk(2)=yk(1)+.alpha..multidot.ek(1) EQU ek(2)=dk(2)-yk(2) EQU yk(3)=yk(2)+.alpha..multidot.ek(2) EQU ek(3)=dk(3)-yk(3)
If the synchronous adaptive noise canceling means 1 carries out the similar processing for the noise component at each of the other phases of the periodic noise, the noise component y(t) produced by the synchronous adaptive noise canceling means 1 finally converges the real signal component included in the target signal e(t) (in the case shown in FIGS. 2(A) through 2(C), the real signal component is "0"). It should be noted that the above-identified reference pulse signal X(t) is used to define each phase of the periodic noise. In addition, with stability and responsiveness of the synchronous adaptive noise canceling means 1 in the process of sequentially producing the noise components taken into account, the above-described correction coefficient .alpha. is appropriately set in a range between zero and one, i.e., 0&lt;.alpha.&lt;1.
On the other hand, for example, in an automatic transmission of an automotive vehicle, means for detecting a torque of an output shaft of the transmission and other means for controlling a torque converter in the transmission on the basis of the detected information of the torque by the above-described means have been developed in order to reduce torque variations on the output shaft of the transmission when a gear rotation ratio is switched in the transmission. The above-described torque detecting means and torque converter controlling means are exemplified by a Japanese Patent Application Unexamined Open No. Sho 50-117479 published on Sept. 13, 1975.
FIG. 3 diagrammatically shows a construction of the automatic transmission and above-described two means.
As shown in FIG. 3, the automatic transmission 10 includes the torque converter 11 through which an engine power is transmitted to an input shaft 12 with the engine torque multiplied. A driving force applied to the input shaft 12 is transmitted to an output shaft 14 with a rotation ratio and rotational direction set in an auxiliary transmission 13. When the output shaft is driven, a propeller shaft 15 is rotated. A magnetostrictive torque sensor 16 for detecting a torque of the output shaft 14 in the automatic transmission 10 is disposed in the proximity of the output shaft 14 as shown in FIG. 3. The torque sensor 16 magnetizes alternatingly the output shaft 14 and detects a change in a magnetic field as the variation of the torque imposed on the output shaft 14.
An output primary signal derived from the magnetostrictive torque sensor 16 is, e.g., shown in FIG. 4(A).
As shown in FIG. 4(A), an alternating signal corresponding to the alternating magnetization has an amplitude changing according to the magnitude of the torque. Since the output shaft 14 has a slight eccentricity and a slight dimensional error due to a material applied to the shaft, the output primary signal is periodically varied in synchronization with the rotation of the output shaft 14.
A signal processing unit 20 eliminates high frequency components of the output signal derived from the torque sensor 16 shown in FIG. 4(A) by means of a low-pass filter after a full-wave rectification of the signal and produces an output signal having a waveform, e.g., as shown in FIG. 4(B). The signal processing unit 20 detects the torque applied to the output shaft 14 on the basis of the output original signal d(t) shown in FIG. 4(B).
A direct current component of the output signal d(t) corresponds to an actual torque on the output shaft 14.
Since the same waveform as shown in FIG. 4(B) is repeatedly formed in synchronization with the rotation of the output shaft 14, the above-described synchronous adaptive noise canceling means 1 may conceivably be applied to eliminate such periodic variations.
Specifically, e.g., as shown in FIG. 3, an electromagnetic induction type rotation sensor 18 is opposingly disposed in the proximity of a surface of teeth on a parking gear 17 utilizing the parking gear 17 having a predetermined number of teeth and installed on the output shaft 14. The reference pulse signal X(t) is thus produced on the basis of the output signal derived from the rotation sensor 18.
The synchronous adaptive noise canceling filter (means) incorporated in the signal processing unit 20 receives the original signal d(t) shown in FIG. 4(B) derived on the basis of the detection signal from the magnetorestrictive torque sensor 16 and reference pulse signal X(t) to produce the target signal.
In a case when a signal such that the direct current component shown in FIG. 4(B) is a real signal on which the periodic noise component is superposed is subjected to the noise cancelation, the above-described synchronous noise canceling (filter) means 1 cannot extract the real signal component from the original signal to be processed including the periodic noise component.
This is because the conventional synchronous adaptive noise canceling means uses the target signal e(t) derived in the process of the noise elimination as a signal contributing the production of a new noise component, the noise components are sequentially produced in accordance with such an equation as yk(i)=yk(i-1)+.alpha..multidot.ek(i-1), and even the direct current component is also eliminated.
Specifically, in the case where the original signal d(t) is as shown in FIG. 5 (substantially the same as the waveform shown in FIG. 4(B)), the noise component yk(i) produced at the predetermined phase k gradually approaches the signal dk(i) to be processed in accordance with the above-described following equation: EQU yk(i)=yk(i-1)+.alpha..multidot.ek(i-1)
Accordingly, since yk(i).apprxeq.dk(i), the signal ek(i) derived from the equation: ek(i)=dk(i)-yk(i) approaches "0" (zero) level.
Consequently, the real signal component cannot be obtained from the original signal to be processed.